The above is simply and introduction. For more information, consult the homepage. The homepage has a lot more information which this article unfortunately doesn't provide. Note that the Invisible IRC Project has evolved into I2P.

I2P is being developed and is currently has some bugs that are causing poor performance. The developers are releasing new versions at about two week intervals. I2P has worked reasonably well in the past so it likely that the problems will be fixed soon.

The developers don't want the project to be widely publicised at the moment.

Services such as email, IRC and access to websites are available within the i2p network.
The i2p router provides anonymised tunnels between users and servers. The ip addresses of peers in the i2p network are visible but in theory it should be difficult for anyone to work out what peers are doing. People can run webservers which are accessable over i2p. In theory it is impossible for the operator of a webserver to work out who is browsing his website and difficult for users to work out which peer in the i2p network is hosting content.

The i2p project does not aim to give perfect anonymity instead balancing performance and anonymity.
Issues such as popular client software leaking information are still being found and the network is small so people requiring high anonymity should not rely on i2p at this time.

I2P differs significantly from freenet in that web pages are only accessable when the server hosting the content is online. Work on a distributed content system seems to have stalled but is likely to resume when the network is working well again.

Some of the i2p services are available from the world wide web because people have set up gateways, such access is unencrypted and could be logged by the gateway operator.
http://i2p.tin0.de/
Note that this is likely to be unreliable at the moment.

There is a freenet node with a large datastore running most of the time which can be accessed over i2p. This provides much better access to freenet than a freenet node which has only been running for a few days.